From the ATP:
This is the 10th straight season that the year-end World No. 1 ranking has been held by Nadal (2008, ’10, ‘13), Federer (2004-07, ’09) or Novak Djokovic (2011-12).
Nadal returned to World No. 1 on 7 October for the first time in more than two years after reaching his 13th final of a phenomenal season at the China Open in Beijing. He made his comeback in February with a No. 5 ranking following a seven-month injury layoff and won 10 titles, including Roland Garros, US Open and five ATP World Tour Masters 1000 triumphs. He also went undefeated on hard courts (26-0) through the China Open final, when he finished runner-up to Djokovic.
Nadal first attained the No. 1 ranking on 18 August 2008 for a period of 46 weeks, becoming the first Spaniard to finish a season at the summit of men’s professional tennis. He regained the top spot from Federer on 7 June 2010, for another 56-week stretch, before relinquishing No. 1 to Djokovic following the 2011 Wimbledon final. Nadal competes in London at No. 1 for the 107th week in his career.Nadal has won 10 titles in 2013 and is currently 73-6. He has never won the ATP Year-End Championship but reached the final in 2010.